1. Field of the Invention
The present invention relates to digital color encoders and, more particularly, to a digital color encoder which, connected to a color television set, converts RGB signals into a composite video signal using a digital system. The present invention also relates to a digital color encoder in which cross color is prevented from occurring even when the one-dimension Y/C separation is conducted at a receiving end of a television signal.
2. Description of the Prior Art
A conventional digital color encoder is constructed as shown in FIG. 1. The digital color encoder comprises a matrix circuit 11 for blending RGB signals sampled at a sampling frequency which is an integral multiple of an integral fraction of a chrominance subcarrier frequency, the blending being done according to a predetermined ratio assigned to each color component. The matrix circuit 11 produces a luminance signal Y and color difference signals R-Y and B-Y. The digital color encoder also comprises low-pass digital filters (LPF) 12-1 and 12-2 for serving as band-reject filters, and a delay circuit (DLY) 13 for delaying the phase of the luminance signal Y by the same interval as a group phase delay interval provided by the low-pass filters 12-1 and 12-2. The distal color encoder also comprises a digital modulator 15 for subjecting the chrominance subcarrier to quadrature two-phase balanced modulation using the color difference signals output by the low-pass filters 12-1 and 12-2 and for outputting a chrominance signal 14; and an adder 18 for computing a sum of the luminance signal delayed by the delay circuit 13, the chrominance signal 14 output by the digital modulator 15, and a composite synchronizing signal 16 that conforms to the standard so as to produce a composite video signal 17. A digital-to-analog converter (D/A) 200 is responsible for subjecting the produced video signal 17 to digital-to-analog conversion.
The digital modulator 15 comprises a cosine table ROM 31 for storing a data table obtained by sampling a cosine waveform ((cos .omega.n.multidot.T), where con indicates an angular frequency and T indicates a sampling cycle) having the same frequency as the chrominance subcarrier (3.58 MHz), using the same sampling frequency as applied to the RGB signals; a sine table ROM 32 for storing a data table obtained by sampling a sine waveform (sin(.omega.n.multidot.T)) having the same frequency as the chrominance subcarrier, using the same sampling frequency as applied to the RGB signals; a multiplier 33 for multiplying an output signal of the low-pass filter 12-1 and an output signal of the cosine table ROM 31; a multiplier 34 for multiplying an output signal of the low-pass filter 12-2 and an output signal of the sine table ROM 32; and an adder 35 for computing a sum of output signals of the multipliers 33 and 34.
As has been described, in the balanced modulation performed by a digital modulator in the conventional digital color encoder, multipliers, a cosine waveform table ROM and a sine waveform table ROM are normally used. The sampling frequency in the balanced modulation is the same as the sampling frequency for the input RGB signals. Accordingly, the multipliers have to be operated at the same frequency as the sampling frequency. Moreover, if a single multiplier is to perform time division process, the single multiplier must be operated at twice the sampling frequency. For example, if the sampling frequency for the input signals (RGB signals) is 13.5 MHz, either two multipliers each operated at 13.5 MHz or a single multiplier operated at 27 MHz are required in order to yield both the R-Y signal and the B-Y signal.
Because high-speed multipliers are indispensable parts of the digital modulator in the conventional digital color encoder, the circuit scale involving the multipliers and the current consumption are inevitably large. Further, because the conventional digital color encoder requires waveform ROM tables adapted for the same sampling frequency as the sampling frequency for the input signals, the capacity of the ROMs and the circuit scale thereof are inevitably large.
A description will now be given, with reference to FIG. 2, of another problem with a conventional digital color encoder. FIG. 2 shows another conventional digital color encoder for producing an NTSC (composite) signal from RGB signals.
A matrix circuit 121 performs matrix operations for blending RGB signals sampled at a predetermined sampling frequency using predetermined ratios assigned to each color component so as to produce a luminance signal Y and color difference signals R-Y and B-Y. The relationship between the luminance signal, the color difference signals and a composite signal M is given by EQU Y=0.30R+0.59G+0.11B) (1) EQU C.sub.v =(R-Y)/1.14 (2) EQU C.sub.U =(B-Y)/2.03 (3) EQU M=Y+C.sub.v .multidot.cos.omega.t+C.sub.u .multidot.sin.omega.t(4)
Low-pass filters (LPF) 123 are responsible for band-rejection for the color difference signals R-Y and B-Y. A modulator 124 performs quadrature two-phase balanced modulation by modulating a chrominance subcarrier (3.58 MHz), using the color difference signal R-Y, and by modulating a chrominance subcarrier having its phase shifted by a phase-shifting device 126, using the color difference signal B-Y so that a chrominance signal is produced. An adder 125 computes a sum of the chrominance signal output from the modulator 124 and the luminance signal adjusted for its timing by a delay circuit 122, so as to produce a video signal.
A receiver that receives the video signal produced by the above-described digital color encoder is required to perform a decoding process called Y/C separation whereby the luminance signal and the chrominance signal are separated from the video signal.
It is known that one-dimensional Y/C separation by which the chrominance signal is separated by allowing the video signal to pass through a band-pass filter does not ensure a high-precision separation in that a component in a luminance signal bandwidth is brought into the chrominance signal in the neighborhood of the chrominance subcarrier frequency, thus causing cross color.
In one alternative Y/C separation (two-dimensional Y/C separation) which relies on a fact that the chrominance signal has alternately opposite phases from line to line, a signal obtained by subtracting a signal for one scanning line from a signal for another is caused to pass through the band-pass filter so as to obtain a chrominance signal in which cross color is reduced. In another alternative Y/C separation (three- dimension al Y/C separation) which relies on a fact that the chrominance signal has alternately opposite phases from frame to frame, a signal obtained by subtracting a signal for one scanning line from a signal for another is caused to pass through the band-pass filter so as to obtain a chrominance signal in which cross color is reduced. However, the circuit constructed according to any of these methods becomes complex and costs a lot.